How to Calculate Extra Money Towards a Mortgage | eHow.comTo calculate the effect of extra payments toward a mortgage, you will need . That represents $39,918 that does not have to be paid, ($665.30 x 60). . Based on the mortgage calculator the amount of finance charges paid will be $110,037.21.http://www.ehow.com/how_6510284_calculate-extra-money-towards-mortgage.html